Sentries (or Pawns) can only move one hex at a time.
They can only move forwards in three possible directions (as illustrated in the diagram) as long as that space is vacant or occupied by an opponent's piece - in which case the captured piece is removed from the board.
Sentries are not able to 'jump' over other pieces.
They can be used to reclaim a captured queen if they reach the opposite side of the board - as in traditional human chess.